2,6-NDC is conveniently formed by the esterification of crude 2,6-naphthalenedicarboxylic acid (2,6-NDA) with methanol. Crude 2,6-NDA typically is contaminated with a variety of by-products such as trimellitic acid (TMA), brominated naphthalene compounds, and 6-formyl-2-naphtbalenecarboxylic acid (FNA). The heavy metals such as cobalt and manganese used to catalyze the oxidation of 2,6-dimethylnaphthalene to 2,6-NDA form insoluble complexes, particularly with the TMA, and typically are included in the crude 2,6-NDA in excess of 1,000 ppm. The insoluble heavy metal complexes form deposits and foul heat exchangers in the esterification process, leading to frequent process interruptions for cleaning and maintenance. It is advantageous to remove the heavy metal contaminants at an early stage in the process to avoid problems in operations.
U.S. Pat. Nos. 5,254,719 and 5,095,135 disclose the use of sulfuric acid as a catalyst to esterify 2,6-NDA to 2,6-NDC. Sulfuric acid is effective at a relatively low temperature, e.g., about 130.degree. C., and it reacts with the heavy metal impurities to form soluble sulfate salts. Disadvantages of sulfuric acid-catalyzed esterifications include corrosion of the reactor and the yield loss of methanol to dimethyl ether. Disposal of waste sulfates is another problem with the use of sulfuric acid catalysts. In addition, the reaction usually is carried out well under the melting point of 2,6-NDC (about 200.degree. C.) to minimize or avoid the above mentioned problems with corrosion and dimethyl ether formation. Thus, the reaction is run in methanol solvent, resulting in a larger vessel for a given residence time relative to a reaction in which the excess methanol is substantially removed in the vapor phase.
U.S. Pat. Nos. 4,003,948 and 5,350,874 and Japanese Unexamined Patent Application (Kokai) 7-233123 disclose processes wherein the esterification is operated at higher temperatures with or without a metallic esterification catalyst. This method has the advantage of reduced dimethyl ether formation and corrosion. Disadvantages include generally higher pressure with no provision to reduce fouling by residual oxidation catalyst metals on heat exchanger or reactor surfaces. Without a provision to recycle incompletely esterified 2,6-NDA to the reactor, it is desirable to run the reaction at a high conversion. In a single backmixed reactor, high conversions are achieved at the expense of increased residence time and larger, more expensive reactors. Methods to narrow the residence time distribution, such as multiple reactors in series or devices to approach a tubular reactor like that disclosed in U.S. Pat. No. 5,350,874, are more expensive and add additional complexity to parts of the process subject to fouling. In the process disclosed in Japanese Unexamined Patent Application (Kokai) 7-233123, the 2,6-NDC is distilled following the reactor and methanol stripper, but the 2,6-NDC distillation is done under vacuum, and there is no provision to recycle partially converted 2,6-NDA. To avoid fouling in the distillation column base heater, it is necessary to dilute the residual oxidation catalyst metals with valuable 2,6-NDC, monomethyl ester of 2,6-naphthalenedicarboxylic acid (2,6-MHN), and 2,6-NDA, thus reducing the yield of 2,6-NDC. It is also necessary to operate the reactor at a high conversion to 2,6-NDC as there is no provision for concentration of the 2,6-NDC in the distillation.
U.S. Pat. No. 3,227,743 addresses some of these problems as they apply to the esterification of terephthalic acid (TPA) with methanol to produce dimethyl terephthalate (DMT) by operating a bubble column reactor under conditions such that the DMT product is removed as a vapor with excess methanol and the water of reaction. The lower vapor pressures of 2,6-DNC and the impurities present in crude 2,6-NDC have until now prevented a direct application to 2,6-NDC. There remains a need in the art for an efficient process for the manufacture of 2,6-NDC that minimizes fouling, operates at high rates, and presents a distilled product free of residual oxidation catalyst metals and other high boilers to a final purification process.